1. Field of the Invention
The invention relates to control systems for navigable craft particularly with regard to flight control systems for aircraft utilizing stability augmentation systems (SAS). More specifically the invention relates to a circuit for automatically recentering the series actuator of the SAS by varying the washout time constant as a function of attitude error.
2. Description of the Prior Art
Prior art automatic pilots are known that utilize stability augmentation systems (SAS) for providing short term stability augmentation about the various axes of the aircraft. Such systems normally utilize limited authority series actuators that operate in response to sensors such as rate gyroscopes or vertical gyroscopes providing derived rate signals. However, as is known, when a craft utilizing such a system executes a maneuver large errors can exist within the stability augmentation system which can cause the series actuator to saturate. This results in an abrupt change in the aircraft's handling characteristics. Although this problem is common to both helicopter and fixed wing stability augmentation systems, changes in handling characteristics are much more noticeable in a helicopter because a helicopter is inherently unstable and difficult to fly. In a helicopter, the stability augmentation system typically relies on a signal from the cyclic pitch stick position transducer and from a vertical gyroscope to drive a limited authority series actuator (short throw hydraulic or electric servo). The series actuator makes minor corrective adjustments to the swash plate in order to stabilize the aircraft. The authority of the actuator and thus the stabilizing adjustments to the swash plate are typically on the order of .+-.10% of the cyclic pitch stick's authority, thus it is desirable that the series actuator remain relatively centered in its range of authority. This is not always possible as the pilot may initiate a trim change greater than the authority of the actuator thus rendering the actuator grossly uncentered and no longer within its symmetrical operating range.
It is known in the art to provide a washout circuit for generating a signal to drive the series actuator back into its centered or symmetrical operating range. The conventional washout circuit operates with a relatively slow time constant, since it is generally undesirable to have the washout circuit attempting to washout or compensate for small high frequency displacements of the series actuator generated in compensating response to air turbulence. After large trim changes or other gross maneuvers, it is necessary to quickly recenter the series actuator by placing the actuator drive circuitry in a fast washout condition. In the prior art, this was done by manually depressing a force trim release button located on the cyclic pitch stick, or by tripping detent switches which triggered the fast washout condition. The transition from slow to fast washout was often abrupt, reflected in radical changes in aircraft handling characteristics. In the fast washout condition, much shorter time constants were used so that the series actuator was quickly centered, however, the aircraft also became less stable.
In addition to triggering the fast washout condition, the force trim release button also actuated a clutch for recentering the conventional feel spring mechanism to its zero force position. The feel spring mechanism is used to impart artificial manual flight control reactions to the human pilot via the stick, as is well known. A problem with this arrangement is that when hovering, particularly at very low altitudes, it may be desirable to fly with the feel spring mechanism disengaged to improve the pilot's reaction time. But with the feel spring mechanism disengaged, it is no longer convenient to use the force trim release button to actuate the fast washout condition. Thus there is a need for a washout circuit which operates automatically and not associated with the force trim release mechanism. Furthermore, it is desirable that the washout make a gradual transition from slow to fast so that abrupt changes in aircraft response are eliminated.